The summing OP-amp could have an infinite number of inputs if you ignore problems with noise an linearity. Each input either adds zero or one quantum. The two comparators checks if the output of the OP-amp is below one quantum or above one quantum. With 64 input bits, you would want your two comparators to verify that the output value is > 1/128 and < 3/128 of the full output of the summing OP when it has all inputs high. With 128 input bits, you would modify the resistor value to add half as much, but instead add 64 more signals. Then change the comparator levels to 1/256 and 3/256.

The problem with the analog solution is speed for quickly changing data, when the input data steps from zero bits set to multiple bits set, or from multiple bits set down to zero bits set., but then digital solutions also often have problems with racing conditions.